1. Technical Field of the Invention
This invention relates to cellular radio telecommunication systems and, more particularly, to a system and method of for restricting the mobility of subscribers assigned to fixed subscription areas (FSAs) in a cellular telecommunications network.
2. Description of Related Art
As the use of cellular telephones spreads, the number of network operators who compete to provide telecommunication services to subscribers is also increasing. Each operator, moreover, desires to offer unique services which distinguish the operator from its competitors. One such way to distinguish the service provided to subscribers is to offer increased levels of individual service tailoring. Today, only limited tailoring of individual services has been implemented. For example, operators may now offer subscribers a choice of subscription areas such as having cellular service only in a downtown area, or for a higher cost, having service which includes both the downtown area and the suburbs as well. Subscribers with geographically limited access rights are referred to as "fixed subscribers" and are generally equipped with either a fixed cellular terminal or a mobile station.
A major problem with existing methods of limiting individual service to a geographic area is that the limits are only effective at system access, i.e., call origination or termination. Existing cellular systems perform a verification process to determine if a subscriber is authorized to place or receive a call from the cell in which the subscriber is located, but the verification process is only performed at system access. Existing systems do not perform subscriber verification at handoff; therefore, once a fixed subscriber places a call in an authorized geographic area, the subscriber may continue the call, even if the subscriber roams outside the authorized area. Thus, once a call is in progress, a fixed subscriber may roam from cell to cell throughout the entire coverage area of the network.
System operators obviously lose revenue when fixed subscribers pay a lower fee for a restricted subscription area and then roam freely into unauthorized areas once calls are begun. However, this unauthorized roaming also causes another serious problem. The cellular system is programmed with information that indicates that a fixed subscriber is limited to operating within the subscriber's restricted or fixed subscription area (FSA). For purposes of the present invention, an FSA is defined as one or more regions of cells within which fixed subscribers are allowed to roam and receive and place calls. The cells may all be of equal size, or may be of different sizes. When a voice channel is assigned to the fixed subscriber, the cellular system may also assign the same channel or adjacent channels to subscribers in areas outside the fixed subscriber's FSA. If the fixed subscriber remains within the authorized FSA, there is normally no problem. However, if the fixed subscriber roams outside the authorized FSA, there is a higher chance of co-channel and adjacent channel interference between authorized subscribers in that area and the unauthorized fixed subscriber.
For a cellular system to control both fixed subscribers and mobile subscribers, the system must be able to identify each type of subscriber and access information regarding each fixed subscriber's authorized FSA. European Patent Application 0 505 106 A2 by Vodafone (Vodafone) utilizes the "classmark" (RF transmitter power) of the subscriber's mobile station to distinguish a regular mobile station from a microcellular mobile station. In this manner, the Vodafone system is alerted if a microcellular subscriber attempts to access the network in a cell which is not a microcell. However, the Vodafone method of utilizing the classmark is useless when the cells in the FSA and bordering cells are of equal size.
Additionally, when required, cellular systems must be prohibited from handing off fixed subscribers to cells outside their FSA. For fixed subscribers with subscription areas comprising a single cell, this means the system may not allow handoffs of those subscribers at all. The cellular systems must, at the same time, allow handoffs of fixed subscribers between authorized cells in FSAs comprising more than one cell, while prohibiting handoff to cells outside the FSA. Meanwhile, normal (unrestricted) mobile subscribers must be able to roam freely to any cell of their choosing. To avoid co-channel and adjacent channel interference between single-cell fixed subscribers, multi-cell fixed subscribers, and mobile subscribers with unrestricted mobility, a mechanism is needed to limit the mobility of fixed subscribers after a call is initiated.
It would be a distinct advantage to have a cellular radio telecommunication system with the capability to identify and drop calls from fixed subscribers who travel out of their authorized FSAs after a call is initiated, and enter cells of equal size. Additionally, it would be advantageous for such a cellular system to provide a warning to fixed subscribers during the call that their call may be dropped. The present invention provides such a system.